1. Field of the Invention
The invention relates generally to downhole tools used to enlarge wellbores drilled in earth formations. More specifically, the invention relates to a fixed blade fixed cutter hole opener having an advanced cutting structure and gage configuration.
2. Background Art
Polycrystalline diamond compact (PDC) cutters have been used in industrial applications including wellbore drilling and metal machining for many years. In these applications, a compact of polycrystalline diamond (or other superhard material such as cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide, to form a cutting structure. A compact is a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass.
An example of a use of PDC cutters is in a drill bit for earth formation drilling is disclosed in U.S. Pat. No. 5,186,268. FIG. 1 in the '268 patent shows a cross-section of a rotary drill bit having a bit body 10. A lower face of the bit body 10 is formed to include a plurality of blades (blade 22 is shown in FIG. 1) that extend generally outwardly away from a rotational axis 15 of the drill bit. A plurality of PDC cutters 26 are disposed side by side along the length of each blade. The number of PDC cutters 26 carried by each blade may vary. The PDC cutters 26 are brazed to a stud-like carrier, which may also be formed from tungsten carbide, and is received and secured within a corresponding socket in the respective blade.
When drilling a wellbore, a PDC bit is attached to the end of a bottom hole assembly (BHA) and is rotated to cut the formations. The PDC bit thus drills a wellbore or borehole having a diameter generally equal to the PDC bit's effective diameter. During drilling operations, it may be desirable to increase the diameter of the drilled wellbore to a selected larger diameter. Further, increasing the diameter of the wellbore may be necessary if, for example, the formation being drilled is unstable such that the wellbore diameter changes after being drilled by the drill bit. Accordingly, tools known in the art such as “hole openers” and “underreamers” have been used to enlarge diameters of drilled wellbores.
In some drilling environments, it may be advantageous, from an ease of drilling standpoint, to drill a smaller diameter borehole (e.g., an 8½ inch diameter hole) before opening or underreaming the borehole to a larger diameter (e.g., to a 17½ inch diameter hole). Other circumstances in which first drilling smaller hole and then underreaming or opening the hole include directionally drilled boreholes. It is difficult to directionally drill a wellbore with a large diameter bit because, for example, larger diameter bits have an increased tendency to “torque-up” (or stick) in the wellbore. When a larger diameter bit “torques-up”, the bit tends to drill a tortuous trajectory because it periodically sticks and then frees up and unloads torque. Therefore it is often advantageous to directionally drill a smaller diameter hole before running a hole opener in the wellbore to increase the wellbore to a desired larger diameter.
A typical prior art hole opener is disclosed in U.S. Pat. No. 4,630,694 issued to Walton et al. The hole opener disclosed in the '694 patent includes a bull nose, a pilot section, and an elongated body adapted to be connected to a drillstring used to drill a wellbore. The hole opener also includes a triangularly arranged, hardfaced blade structure adapted to increase a diameter of the wellbore.
Another prior art hole opener is disclosed in U.S. Pat. No. 5,035,293 issued to Rives. The hole opener disclosed in the '293 patent may be used either as a sub in a drill string, or may be coupled to the bottom end of a drill string in a manner similar to a drill bit. This particular hole opener includes radially spaced blades with cutting elements and shock absorbers disposed thereon.
Other prior art hole openers include, for example, rotatable cutters affixed to a tool body in a cantilever fashion. Such a hole opener is shown, for example, in U.S. Pat. No. 5,992,542 issued to Rives. The hole opener disclosed in the '542 patent includes hardfaced cutter shells that are similar to roller cones used with roller cone drill bits.
There is a need, however, for a hole opener that makes use of recent advances in PDC cutter and blade technology. While PDC cutters have been used with, for example, prior art near-bit reamers, the PDC cutters on such reamers are generally arranged in a relatively simplistic fashion. This arrangement, among other factors, forms a relatively unreliable mechanical structure that is not durable, especially when drilling tough formations. Moreover, some prior art hole openers generate high levels of vibration and noise, and tend to cause the well trajectory deviate from the existing well trajectory. Therefore, it would be advantageous to produce hole openers with improved cutting structures.